Help, what is it?

[grant mills]

In my previous query about radio upgrade, I asked about a part that sat on top of the old radio and was connected to an under dash harness (NOT to the radio amp output) I have not heard any explanation for the part. Its base is 4.25" square. There is a trim-able pot. as shown in the first picture and the connector to the under dash harness in the third. The only connection to the old radio itself was a ground wire connected to the rear support strap of the radio. Does anyone have any idea what the purpose is of this simple PCB. What might it control/support?    (Aside: I have installed a "RetroSound" replacement and it fits like a glove in the old opening....I only have to memorise 18 pages of control options/settings)